Soild state protection circuit for electrical apparatus

ABSTRACT

Electrical equipment is safeguarded from damage due to faults by a circuit that provides several levels of protection. A semiconductor switch and a current sensor are placed in series with the electrical equipment. When the current to the equipment exceeds a first threshold for a predefined period of time, the semiconductor switch is rendered non-conductive until the circuit is specifically reset. When the current to the equipment exceeds a greater second threshold, a pulsed signal alternately places the semiconductor switch in conductive and non-conductive states so that the average current applied to the equipment is within an acceptable level. Should the current exceed an even greater third threshold, the semiconductor switch is immediately rendered non-conductive until the circuit is manually reset.

BACKGROUND OF THE INVENTION

The present invention relates to mechanisms that protect equipment fromdamage due to electrical faults and short circuits; and particularly tosuch devices which electronically monitor performance of the electricequipment and take protective action in the event of a fault, short oroverload.

It is important that electrical apparatus be protected from damage whenelectrical failures occur. For example, conventional fuses andelectromechanical circuit breakers are commonly employed to disconnectequipment from an electrical supply upon detection of excessive currentwhen a short circuit occurs. Nevertheless, these conventional protectiondevices are relatively slow in disconnecting the current flow to theapparatus being protected. As a consequence, enough excessive electricalcurrent can flow into the equipment to cause damage during a fault.

Furthermore, various electrical apparatus require different responsecharacteristics for the protection device. For example, electronicequipment may draw a substantially constant current level from initialstart-up through a normal shut-down and be very intolerant of excessivecurrent levels of even short duration. The protection device for suchequipment has to respond very quickly to even relatively small overcurrent conditions. Other types of electrical equipment draw largeinstantaneous current levels at certain times, such as upon start-up, incomparison to the current level drawn during remainder of theiroperation. Thus, a circuit protection device that responds too rapidlyto an high current condition may inadvertently shut-off current to theequipment during normally occurring events. As a consequence, theprotection device for this type of equipment must respond in a mannerthat tolerates brief high currents. The manner in which a protectiondevice responds to over currents is referred to as the trip responsecharacteristic or trip curve, and has to be matched to the particulartype of electrical apparatus being protected.

This usually means that a manufacturer of protection devices mustdesign, manufacture and stock in inventory, a large variety ofprotection devices that have different trip response characteristics interms of current level and duration. Thus it is desirable to provide abasic configuration of a protection device which can be customizedeasily with different trip response characteristics.

SUMMARY OF THE INVENTION

An apparatus for protecting an electrical load from excessive currentemploys a semiconductor switch to connect the electrical load to asource of current. A current sensor is coupled in series with thesemiconductor switch and produces a sensor signal that indicates themagnitude of current flowing to the electrical load.

A control circuit is connected to the current sensor and thesemiconductor switch. The control circuit responds to the sensor signalby producing a control signal that is applied to a control input of thesemiconductor switch. In a first mode of operation when the magnitude ofcurrent is less than a first threshold, the control circuit maintainsthe semiconductor switch in a continuous conductive state. When themagnitude of current is greater than the first threshold and less than asecond threshold, the control circuit in a second mode of operationrenders the semiconductor switch non-conductive after a predefinedperiod of time. In a third mode of operation when the magnitude ofcurrent is greater than the second threshold, the semiconductor switchis alternately pulsed conductive and non-conductive by the controlcircuit to apply an average current through the load that is within anacceptable level wherein damage does not occur.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a solid state circuit protector accordingto the present invention;

FIG. 2 illustrates the details of an instant trip circuit in theprotector; and

FIG. 3 is a graph of an exemplary trip response characteristic of thesolid state circuit protector.

DETAILED DESCRIPTION OF THE INVENTION

With initial reference to FIG. 1, a solid state circuit protector 10controls application of a direct current to an electrical load 14,depicted as capacitance in parallel with a resistance. The solid statecircuit protector 10 has a positive voltage terminal 12 which isconnected to the electrical source for powering a load 14. Current flowsfrom the positive voltage terminal to the load through a back-up fuse16, a semiconductor switch 18 and an inductor 20 to a load terminal 22.The load is connected between the load terminal 22 and the negative sideof the voltage supply, represented as ground.

The back-up fuse 16 is a conventional device with a conductor whichheats-up and ultimately breaks when excessive current flows for a givenperiod of time. Standard devices, such as glass tube encased fuses or anappropriate trace on printed circuit board, can be employed for theback-up fuse 16. The back-up fuse 16 provides redundant protection incase the semiconductor switch 18 fails in the conductive state or uponfailure of electronic circuits controlling the semiconductor switch. Aswill be understood, the trip response time of the back-up fuse isconsiderably slower than the trip response characteristic of theelectronic circuit protection.

The semiconductor switch 18 must be able to interrupt the load currentand handle transient currents, over currents and in-rush at a specifiedoperational voltage range as dictated by the particular load 14 to becontrolled. An n-channel field effect transistor (FET), such as modelIRF1404 from International Rectifier of El Segundo, Calif. 90245 USA maybe used as the semiconductor switch 18. The channel resistance in theconductive state has to be relatively low to minimize the voltage dropacross the FET and the heat dissipation. Although the preferredembodiment employs the semiconductor switch 18 between the positivevoltage terminal 12 and the load 14, alternatively the switch could beplaced on the ground side of the load. However, this alternativeapproach has the disadvantage that a fault from load to ground would beunprotected.

A voltage sensor 28 produces an analog signal which indicates thevoltage level at the load terminal. That analog signal is applied to ananalog input of a microcontroller 26. As will be described, themicrocontroller 26 responds to an indication from the sensor 28 that thevoltage across the load 14 is too low by turning off the semiconductorswitch 18.

A current sensor 24 is provided to detect the level of current flowingbetween the positive voltage terminal 12 and the load 14. This sensormust have a dynamic range which is large enough to cover the currentextremes for the desired trip response characteristic of the protectiondevice and have a transient response that is sufficiently fast toimplement the desired trip response characteristic. The current sensor24 may be a Hall effect sensor that produces an output voltageindicative of the DC current magnitude and which output voltage can beapplied via line 31 directly to an analog input of a microcontroller 26.Other types of conventional current sensors, such as a shunt resistor,may be used to provide a current magnitude indication to themicrocontroller 26.

The microcontroller 26 is microprocessor based and includes an internalanalog-to-digital converter with a multiplexed input for signals fromthe current and voltage sensors. Digital input/output circuits of themicrocontroller handle signals for other components of the solid statecircuit protector 10. For example, a user control panel 25 has a keypad27 and light emitters 29, such as LED's. The keypad 27 has separatemomentary contact switches that supply input signals to themicrocontroller 26 to manually turn the solid state circuit protector 10on and off, as well as reset a trip condition. The light emitters 29 arepowered by signals from the microcontroller to indicate the operationalstates of the circuit protector. One of those light emitters 29indicates when the circuit protector 10 is tripped. The microcontroller26 also has an internal non-volatile memory which stores a softwareprogram defining the protection function and which stores data, such asthe trip response characteristic, for use by that software program. Themicrocontroller 26 and the control panel 25 optionally can controladditional poles of a circuit protector as indicated by a second pole 11drawn in phantom lines.

The microcontroller 26 operates the semiconductor switch 18 through atrip circuit 36 that generates a drive voltage which is adequate tocontrol the FET 19 in the preferred embodiment of the semiconductorswitch 18. Because the voltage driving the gate of an N-channel FET 19has to be approximately ten volts greater than the voltage at the sourceelectrode of the FET, the trip circuit 36 includes a charge pump orsimilar circuit to generate voltage greater than that found on thepositive input terminal 12.

FIG. 2 illustrates the details of the trip circuit 36 wherein the outputsignal I_(SENSE) on line 31 from the current sensor 24 is applied to afirst voltage comparator 40. The sensed current level I_(SENSE) iscompared to a second threshold I_(TH2) which is produced on an analogoutput line 37 of the microcontroller 26. A fixed value for the secondthreshold I_(TH2) is programmed into the microcontroller 26 dependingupon the over current tolerance of the specific load 14. The result ofthat comparison at the output of the first comparator 40 is applied tothe RESET input of a flip-flop 42. The reset input also is connected toa positive supply voltage V⁺ by a pull-up resistor 44.

The SET input of the flip-flop 42 is connected to the output of a dualinput NAND gate 46, having both inputs tied together to function as aninverter. The inputs of first NAND gate 46 are connected to a digitaloutput line 33 from the microcontroller 26 which carries a pulsed signalat a fixed frequency in excess of 15 kHz., specifically in the range of20-30 kHz. and preferably at 25 kHz. The pulsed signal has a fixed dutycycle thereby forming a train of constant-width pulses. As will bedescribed, the pulse train periodically sets the flip-flop output whichis tied to one input of a second NAND gate 48 having three inputs.Another input of the second NAND gate 48 receives an ON signal onanother digital output line 33 from the microcontroller 26. Whether theON signal is active or inactive is determined by manual operation ofswitches on keypad 27 of the control panel 25.

The third input of the second NAND gate 48 receives an output signalfrom an instant trip mechanism formed by a second voltage comparator 50and a second flip-flop 51. Specifically, the second comparator 50compares the current sensor output signal I_(SENSE) to a third thresholdI_(TH3). The third current threshold I_(TH3) is generated on anotheranalog output line 38 by the microcontroller 26 and is defined by afixed value programmed into the solid state protection circuit 10. Thethird current threshold I_(TH3) is greater than the second currentthreshold I_(TH2). The precise relationship between those two currentthresholds will become apparent from a subsequent description of theoperation of the solid state current protection circuit. The second andthird current thresholds I_(TH2) and I_(TH3), instead of beingprogrammable, may be set by conventional voltage dividers at the inputsto the respective comparators 40 and 50. The output of the secondcomparator 50 is latched by the second flip-flop 51 with an outputconnected to another input of the second NAND gate 48. The set input ofthe second comparator 50 is connected to RESET output line 35 from themicrocontroller 26.

The components of the trip circuit 36 described thus far, provide inputsignals to the second NAND gate 48. The output of that gate is fedthrough a third NAND gate 52 which is connected as an inverter. Thesignal emanating from the third NAND gate 52 is coupled by a resistor 56to an isolation circuit 54, such as a standard opto-isolator. Theisolation circuit 54 produces an output on line 58 that is applied to aninput of a conventional FET gate driver circuit 60. A charge pump 62provides a voltage level that the FET gate driver 60 uses to bias thegate of the FET 19 via line 39.

The operation of the solid state circuit protector 10, in FIG. 1,commences with the operator pressing an appropriate switch on keypad 27.The microcontroller 26 responds to this switch activation by applying ahigh level, or active ON signal, via line 34 to the second NAND gate 48.At this time, the microcontroller 26 also begins producing a pulse trainon digital output line 33 connected to the first NAND gate 46. A highlogic level of that pulse train causes the output of the first flip-flop42 to go high, applying another high level to another input of thesecond NAND gate 48.

During normal operation of the load 14, the output signal I_(SENSE) fromthe current sensor 24 is less than the third threshold I_(TH3). As aconsequence, the second voltage comparator 50 produces a high logiclevel at the third input of the second NAND gate 48. Thus the secondNAND gate 48 produces a low level output signal that, upon inversion bythe third NAND gate 52 and conduction through isolator 54, activates theFET gate driver 60. This causes the gate driver 60 to bias the gate ofthe FET 19 into a conductive state, thereby applying current from thepositive voltage terminal 12 through the inductor 20 to the load 14.

The level of current through the semiconductor switch 18 rises rapidlyand soon exceeds the second threshold I_(TH2). At that time, the outputof the first comparator 40 goes low resetting the flip-flop 42 andcausing the second NAND gate 48 to change output states. This results inthe FET gate driver 60 rendering the semiconductor switch 18non-conductive. The energy stored in the inductor 20 produces a decayingcurrent that flows through the load 14 and the fly back diode 21.

When the next positive pulse occurs in the pulse train on line 33 to thefirst NAND gate 46, the flip-flop 42 will be SET to produce another highlogic output level which once again turns on the FET gate driver 60 andthe semiconductor switch 18. This on-off cycling of the semiconductorswitch continues chopping the current at the rate of the signal on line33 until the capacitance in the load 14 adequately charges, at whichtime the load current becomes substantially constant at a level lessthan the second threshold I_(TH2). Thus the load current during start-upis limited to being less than the second threshold I_(TH2) while stillapplying current to initialize the load operation. Once the excursionsof current through the semiconductor switch 18 fall below thisthreshold, the flip-flop 42 no longer is reset and the FET gate driver60 maintains the semiconductor switch 18 in a conductive state. Thatconductive state continues as long as the load 14 functions normally.

If there is a fault with the load during start-up, the load current doesnot drop below the second threshold I_(TH2). The current chopping couldcontinue indefinitely in this case. To prevent that, the duration of thecurrent chopping is limited by counting the current pulses applied tothe load and terminating the chopping upon the occurrence of givennumber of pulses that normally is sufficient to charge a typical loadcapacitance. Specifically, the microcontroller 26 monitors the inputline 31 from the current sensor 24 which indicates alternate highcurrent and zero current conditions and counts the number of highcurrent pulses. That count is compared to a reference number and thechopping mode is terminated when that reference number of current pulseshas occurred. At that time, the microcontroller 26 sends a low logiclevel signal on line 34 to the trip circuit 36, which renders thesemiconductor switch non-conductive until a person presses the RESETswitch on the control panel 25 and resets the microcontroller.

Alternatively the voltage sensor 28 can be employed to safeguard againstoperating in the current chopping mode for too long a time period.During a short circuit condition when the load 14 is drawing excessivecurrent, the voltage across the load will be significantly lower thanduring normal operation. The voltage across the load 14 is detected bythe voltage sensor 28 which applies an analog voltage level indicationto the microcontroller 26. If that sensed load voltage remains below agiven threshold for greater than a predefined time interval during thecurrent chopping mode, the microcontroller 26 turns off the trip circuit36 by applying a low logic level, (an inactive ON signal) to the ON/OFFline 34.

The operation of the solid state protection circuit 10 during an overcurrent condition after a normal start-up may best be understood withrespect to an exemplary trip response characteristic, such as the onedepicted in FIG. 3. A load current which is below a first thresholdI_(TH1) can be tolerated indefinitely by the load 14 and thus will beconducted continuously by the semiconductor switch 18. The firstthreshold I_(TH1) is set between 100% and 125% of the current rating forthe load 14 being protected. Load currents between levels I_(TH1) and I2can be tolerated by the load for an amount of time which is inverselyproportional to the current magnitude. In other words, small deviationsabove the first threshold I_(TH1) can be tolerated for a longer periodof time than over currents which approach level I2. This produces alinear trip response characteristic in portion 70 of the response curve.This portion of the trip response characteristic is programed into themicrocontroller 26 and stored in its memory either as a linear equationor as a data table. That data table has pairs of values with one valuebeing a current magnitude and the other value defining a time intervalduring which that current magnitude can be tolerated before the solidstate circuit protector 10 must trip.

Current between level I2 and the third threshold I_(TH3) can betolerated by the load for a period designated T1. Current above thathigher level I_(TH3) cannot be tolerated by the load 14, evenmomentarily, and thus the current protection device will tripimmediately. It should be noted, that load current within thecross-hatched region 72 between second threshold I_(TH2) and a thirdthreshold I_(TH3) while tolerated by the load 14, can damage the FET 19.Thus, when operation within this region is determined to occur, thesolid state circuit protector 10 enters a current chopping mode ofoperation. In this mode the semiconductor switch is pulsed on and off ata rate which produces an average current that is less than the secondthreshold I_(TH2). Thus the load remains powered so that the loadcapacitance remains charged but the current applied to the load islimited to that second threshold level.

When the sensed current I_(SENSE) is between the first threshold I_(TH1)and a second threshold I_(TH2), the trip circuit 36 initially maintainssemiconductor switch 18 in a conductive state because that current isbelow the two comparator thresholds I_(TH2) and I_(TH3). However, themicrocontroller 26 receives the output signal I_(SENSE) from the currentsensor 24 on line 31 utilizes the programed trip response characteristicfor section 70 to determine whether to turn-off the semiconductor switch18. Specifically, the microcontroller 26 determines whether the overcurrent magnitude has occurred for the time period defined by the tripresponse characteristic. Once that has occurred, the microcontroller 26turns off the trip circuit 36 by applying a low logic level, inactive ONsignal, to the digital line 34. This constant low logic level togglesthe output level from the second NAND gate 48 which turns-off the FETgate driver 60 and thus the semiconductor switch 18. The microcontroller26 also illuminates the light emitter 29 on the control panel 25 whichindicates the tripped condition. The OFF signal continues to be appliedby the microcontroller 26 to the trip circuit 36 until a manual resetswitch on the control panel 25 pressed.

When the sensed load current signal I_(SENSE) is between the currentthresholds I_(TH2) and I_(TH3), the microcontroller 26 does not utilizethe trip response characteristic data to determine whether to turn-offthe trip circuit 36. Instead, the solid state protection circuit 10enters a current chopping mode in which the FET 19 is pulsed on and offat the rate of the pulsed signal on line 33.

Specifically with reference to FIG. 2, when the current sensor 24produces an output signal I_(SENSE) on line 31 which is greater than thesecond threshold I_(TH2) on line 37, the output of first comparator 40goes low. That low output resets the flip-flop 42, thereby applying alow logic level to an input of the second NAND gate 48. This produces ahigh logic level at the output of the second NAND gate 52 which isinverted by the third NAND gate 52 thus applying a low logic level tothe opto-isolator 54. This in turn deactivates the FET gate driver 60which renders the semiconductor switch 18 nonconductive. At that timecurrent from the inductor 20 flows through the load 14 and a fly backdiode 21.

The semiconductor switch 18 remains off until the next high logic levelpulse in the pulse train from the microcontroller 26 that is applied tothe trip circuit 36 on digital line 33. That pulse upon inversion by thefirst NAND gate 46 sets the flip-flop 42 which produces a high outputlevel that is applied to the second NAND gate 48. This high logic levelactivates the FET gate driver 60, once again rendering the semiconductorswitch 18 conductive.

When the FET 19 turns on again, the inductor 20 limits the rate at whichthe current rises so that the current level does not immediately exceedthe second threshold I_(TH2). Thus, a small amount of current will beapplied to the load 14 and charging its capacitance. However, thecurrent through the semiconductor switch 18 eventually rises above thesecond current threshold I_(TH2) which will be detected by the firstcomparator 40. When this occurs the first comparator 40 changes outputstates and resets the flip-flop 42, which in turn applies a signal tothe second NAND gate 48 that ultimately results in the FET gate driver60 shutting off the semiconductor switch 18. This cycling of thesemiconductor switch 18 off and on continues which results in an averageload current that is below the first threshold I_(TH1).

Although the semiconductor switch 18 is not exposed to as great a degreeof thermal stress as with linear current limiting, damage to the FET 19or the load still may occur if the current chopping mode continues fortoo long a time period. As discussed previously with respect thestart-up operation of the solid state circuit protector 10, the durationof the current chopping can be limited by the microcontroller 26counting the number of current pulses applied to the load and sending alow logic level OFF signal on line 34 to the trip circuit 36 when agiven number of pulses has occurred. Alternatively the voltage sensor 28can be employed to detect a short circuit and inform the microcontroller26 to turn-off the trip circuit 36.

The current chopping should ensure that the load current never exceedsthe third threshold level I_(TH3). However, in the event that amalfunction occurs, the second comparator 50 detects a load currentabove that third threshold level I_(TH3) and produces an output thatrenders the semiconductor switch 18 continuously non-conductive.Specifically, the output of the second comparator 50 goes low whichresets the second flip-flop 51 thereby applying a low logic level to thesecond NAND gate 48. This results in the FET turning off.

We claim:
 1. An apparatus for protecting an electrical load fromexcessive current, which apparatus comprises: a semiconductor switch forconnecting the electrical load to a source of current and having acontrol input; a current sensor coupled to the semiconductor switch andproducing a sensor signal indicative of a magnitude of current flowingto the electrical load; and a control circuit connected to the currentsensor and the semiconductor switch and responding to the sensor signalby producing a control signal that is applied to the control input, thecontrol circuit having a first mode of operation when the magnitude ofcurrent is less than a first threshold wherein the semiconductor switchis maintained in a continuous conductive state, a second mode ofoperation when the magnitude of current is greater than the firstthreshold and less than a second threshold wherein the semiconductorswitch is rendered non-conductive after a predefined period of time, anda third mode of operation when the magnitude of current is greater thanthe second threshold wherein the semiconductor switch is alternatelypulsed conductive and non-conductive to limit the current through theload to less than the second threshold.
 2. The apparatus as recited inclaim 1 wherein the control circuit remains in the second mode ofoperation until specifically reset.
 3. The apparatus as recited in claim1 wherein the semiconductor switch is alternately pulsed conductive andnon-conductive for cycles having identical periods.
 4. The apparatus asrecited in claim 1 wherein the semiconductor switch is alternatelypulsed conductive and non-conductive at a rate of at least 15 kHz. 5.The apparatus as recited in claim 1 wherein the semiconductor switch isalternately pulsed conductive and non-conductive at a rate between 20kHz and 30 kHz.
 6. The apparatus as recited in claim 1 wherein thecontrol circuit has a fourth mode of operation when the magnitude ofcurrent is greater than a third threshold that is greater than thesecond threshold, wherein the semiconductor switch is renderednon-conductive by the control signal until the apparatus is specificallyreset.
 7. The apparatus as recited in claim 1 wherein the controlcircuit comprises: a programmable controller which produces a pulsedsignal having a constant duty cycle and an ON signal; a switch driverthat produces the control signal in response to a switch activationsignal; and a trip circuit connected to the programmable controller, theswitch driver and the current sensor, said trip circuit including alogic circuit that produces the switch activation signal when the sensorsignal is less than the second threshold and the ON signal is active,and that produces a pulsed switch activation signal in response to thepulsed signal when the sensor signal is greater than the secondthreshold and the ON signal is active.
 8. The apparatus as recited inclaim 7 wherein the programmable controller produces an inactive ONsignal in response to the magnitude of current being greater than thefirst threshold for the predefined period of time.
 9. The apparatus asrecited in claim 8 wherein the programmable controller stores dataspecifying the predefined period of time as a function of the magnitudeof current.
 10. The apparatus as recited in claim 7 wherein the tripcircuit comprises: a first comparator that compares the sensor signal tothe second threshold and in response produces a first control signal atan output; a flip-flop having one input connected to the output of thefirst comparator and another input connected to the programmablecontroller to receive the pulsed signal, and having an output; and alogic gate having one input connected to the output of the flip-flop andanother input connected to the programmable controller to receive the ONsignal, and having an output coupled to the switch driver.
 11. Theapparatus as recited in claim 10 wherein the trip circuit furthercomprises a second comparator that compares the sensor signal to a thirdthreshold and has an output coupled to the logic gate to render thesemiconductor switch non-conductive when the sensor signal exceeds thethird threshold.
 12. The apparatus as recited in claim 1 furthercomprising a fuse in series with the semiconductor switch.
 13. A methodfor protecting an electrical load from an excessive magnitude ofcurrent, which method comprises: placing a semiconductor switch into aconductive state to apply current from a source to the electrical load;sensing the magnitude of the current; comparing the magnitude of thecurrent to a first threshold; comparing the magnitude of the current toa second threshold that is greater than the first threshold; determiningwhen the magnitude of the current exceeds the first threshold for apredefined period of time and when that occurs issuing a terminationsignal; in response to the termination signal, placing the semiconductorswitch in a non-conductive state to terminate application of current tothe electrical load; and when the magnitude of the current exceeds thesecond threshold, alternately placing the semiconductor switch inconductive and non-conductive states to apply current pulses to theelectrical load and limit the average current through the load to lessthan the second threshold.
 14. The method as recited in claim 13 whereinalternately placing the semiconductor switch conductive andnon-conductive states apply current pulses in cycles having identicalperiods.
 15. The method as recited in claim 13 further comprisesdetermining how long the semiconductor switch is alternately placed inconductive and non-conductive states; and placing the semiconductorswitch in a continuous non-conductive state after a predetermined periodof time.
 16. The method recited in claim 13 further comprising: sensinga magnitude of voltage across the electrical load; comparing themagnitude of voltage to a given threshold; and placing the semiconductorswitch in a continuous non-conductive state, when the magnitude ofvoltage is less than the given threshold for a predetermined period oftime.
 17. The method as recited in claim 13 further comprising:comparing the magnitude of the current to a third threshold that isgreater than the second threshold; and placing the semiconductor switchin a continuous non-conductive state when the magnitude of the currentexceeds the third threshold.
 18. The apparatus as recited in claim 13wherein alternately placing the semiconductor switch conductive andnon-conductive states applies current pulses to the electrical load at arate of at least 15 kHz.
 19. The apparatus as recited in claim 13wherein alternately placing the semiconductor switch conductive andnon-conductive states applies current pulses to the electrical load at arate between 20 kHz and 30 kHz.